Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Stock-based compensation	$ 7,797	$ 7,634
Non-deductible realized loss on investments	2,685	4,708
Dividend equivalents on unvested restricted stock units	2,686	3,052
Deferred Tax Assets, Unrealized Losses on Trading Securities	4,101	4,642
Deferred compensation	(4,528)	(6,792)
Deferred rent	2,407	2,414
Other	(2,743)	(757)
Subtotal	12,405	14,901
Less: valuation allowance	(6,786)	(9,350)
Deferred income tax asset - net	5,619	5,551
Capital Loss Carryforward	6,959	$ 6,413
Changes in Valuation Allowance	$ 2,564